The present invention relates to a method of holding a substrate and a substrate holding system to hold a substrate securely during a production process for treating the substrate, such as semiconductor device, while it is being cooled.
Among substrate treating apparatuses for production of semiconductor devices, there are a lot of substrate treating apparatuses requiring the cooling substrates, such as a plasma treatment apparatus, a sputtering apparatus, a dry etching apparatus, a CVD (chemical vapor deposition) apparatus and a high energy ion implantation apparatus. Since the treating environment in these apparatuses is generally a vacuum, it is difficult to cool a substrate by bringing it into contact with a cooling surface, as in atmospheric pressure, because of the decrease in thermal conductivity which occurs in a vacuum. Although there is abundant literature concerning thermal conductivity in a vacuum (rarefied gas), the amount of heat transferred by contact is small because of the small real contact area when surfaces come into contact with each other. Especially in heat transfer between a substrate and a cooling surface, it is difficult to strongly press the substrate against the cooling surface since there is a possibility to damage the substrate. Therefore, various ideas, such as placing a soft elastomer on the surface contacting the substrate, have been proposed. However, in recent years, it has been more conventional for a gas to be introduced between the substrate and a cooling surface to cool the substrate using the gas as a coolant, when the heat load in the substrate increases or a requirement to cool the substrate to lower the temperature thereof arises.
There are various types of gas cooled substrate holding systems. They can be roughly categorized as follows: (1) a gas cooling type system where the back surface of a substrate and a cooling surface contact each other and a gas is introduced into the gap between the surfaces formed by the surface roughness, and (2) a gas cooling type system where the back surface of a substrate and a cooling surface do not contact each other and a gas is introduced into the gap between the surfaces.
The prior art gas cooling type systems belonging to the former category (1) are described in, for example, Japanese Patent Publication No. 2-27778 (1990), Japanese Patent Application Laid-Open No. 62-274625 (1987), Japanese Patent Application Laid-Open No. 1-251375 (1989), Japanese Patent Application Laid-Open No. 3-154334 (1991) and Japanese Utility Model Application Laid-Open No. 4-8439 (1992). And, the prior art gas cooling type systems belonging to the latter category (2) are described in, for example, Japanese Patent Application Laid-Open No. 63-102319 (1988), Japanese Patent Application Laid-Open No. 2-312223 (1990), Japanese Patent Application Laid-Open No. 3-174719 (1991). Further, there is another type of system, described in Japanese Patent Application Laid-Open No. 2-30128 (1990), where, before introducing a cooling gas, the back surface of a substrate and a cooling surface are brought into contact with each other, but during cooling the substrate is pushed up due to gas pressure caused by introducing the cooling gas and does not contact the cooling surface.
In these cooling systems, providing that a certain cooling gas is used, the cooling capacity (magnitude of transferred heat) with the cooling gas depends on the pressure of the gas and the distance between the back surface of a substrate and the cooling surface (gap in the back surface of the substrate). FIG. 8 schematically shows the characteristic of thermal conductivity in a low pressure situation. When the pressure of the cooling gas is low, the amount of transferred heat is proportional to the pressure of the cooling gas and independent of the magnitude of the gap between both of the surfaces. When the pressure of the cooling gas is higher than the pressure PO, where the mean free path of the cooling gas nearly coincides with the gap, the amount of transferred heat becomes constant and independent of the gas pressure. The pressure of the cooling gas in the type of system in category (1) described above is generally in the region where the heat transfer is proportional to pressure, and the pressure of the cooling gas in the type of system in category (2) described above is generally in the region where the heat transfer is independent of pressure.
Characteristics and problems in various methods of cooling a substrate will be described below.
First, a description will be made on the case where cooling is performed under a condition that a substrate contacts a cooling surface. The cooling methods belonging to this type are disclosed in Japanese Patent Publication No. 2-27778 (1990), Japanese Patent Application Laid-Open No. 62-274625 (1987), Japanese Patent Application Laid-Open No. 1-251375 (1989), Japanese Patent Application Laid-Open No. 3-154334 (1991) and Japanese Utility Model Application Laid-Open No. 4-8439 (1992). In the cooling method of this type, although the substrate and the cooling surface contact each other, only the most protruding portions on the cooling surface contact the substrate when it is observed in detail. The indented portions on the cooling surface and on the substrate do not contact each other, and the gaps are approximately 10 .mu.m to 50 .mu.m, although this depends on the surface roughness. In a case where a cooling gas is introduced in the gap, the pressure is generally several Torrs, which is in a region nearly equal to the mean free path. Therefore, a sufficient cooling efficiency can be obtained by properly setting the pressure as shown in FIG. 8.
However, when the cooling gas is supplied from a specified single portion, as shown in the figure in Japanese Patent Publication No. 2-27778 (1990), the pressure is highest in the cooling gas supplying portion and decreases as it goes toward the peripheral portion of the substrate. Since the cooling efficiency has a pressure dependence as shown in FIG. 8, there arises a disadvantage that uniformity of the temperature distribution is deteriorated due to the non-uniformity of the cooling efficiency. If there is no gas leakage, that is, no gas flow, the pressure distribution does not occur and the temperature distribution becomes uniform. However, in order to achieve this, the peripheral portion of the substrate needs to be shielded. This is described in Japanese Patent Application Laid-Open No. 62-274625 (1987) or in Japanese Utility Model Application Laid-Open No. 2-135140. Further, the method in which cooling gas is supplied from plural portions to make the pressure distribution on the back of the substrate uniform is described in Japanese Patent Application Laid-Open No. 1-251735 (1989) or in Japanese Patent Application Laid-Open No. 4-61325 (1992). In any case, in these cooling methods, since the back surface of the substrate and the cooling surface contact each other in a large area, there is a disadvantage in that a lot of foreign substances become attached to the back surface of the substrate when contacting the cooling surface. Further, in order to prevent the cooling gas from leaking through the peripheral portion of the substrate using a shielding material, a load for the shielding needs to be applied. Therefore, a way of tightly fixing the substrate in some manner is required.
Description will be made below of a cooling method where a substrate and a cooling surface do not contact each other and a cooling gas is supplied into the gap. The prior art method is described in Japanese Patent Application Laid-Open No. 3-174719 (1991) or in Japanese Patent Application Laid-Open No. 4-6270 (1992), in which a substrate is mechanically fixed to a cooling surface from the top surface or the side surface of the substrate. Since the substrate, in these examples, is mechanically fixed, there is a disadvantage that foreign substances are apt to be produced at the fixing portion. In the methods described in Japanese Patent Application Laid-Open No. 63-102319 (1958) and in Japanese Patent Application Laid-Open No. 2-30128 (1990), a substrate is not fixed specially, but is held by the weight of the substrate itself. In this case, in order that the leakage of the cooling gas is not increased too much or the substrate is not caused to float up, the pressure of the cooling gas has to be limited to a low level. This causes a disadvantage in that the cooling efficiency is decreased.
Electrostatic adhesion is a known method of fixing a substrate electrically. An example where a substrate is fixed to a cooling surface with this method and projections are provided on the periphery of the substrate is described in Japanese Patent Application Laid-Open No. 62-208647 (1987). A substrate contacts a cooling surface only at a plurality of projections provided in separate spaced relation on the outer periphery and inner periphery of the substrate, which is described in the Japanese Patent Application Laid-Open No. 62-208647 (1987). And, this publication indicates that cooling gas easily leaks and that the adhering force is unstable. Further, in order to improve this method, it is effective if the outer periphery is not projected and the projections are provided only on the inner peripheral portions, and further, if the projections in the inner periphery are provided in the central portion, instead of in separate spaced relation. In this case, the gap between the substrate and the cooling surface-becomes non-uniform over the surface of the substrate, which causes a non-uniform pressure distribution on the back surface of the substrate. When the gap between the back surface of the substrate and the cooling surface varies from one position to another, the ratio of the mean free path of the cooling gas and the gap has an uneven distribution over the surface of the substrate. Therefore, a disadvantage arises in that the temperature distribution is apt to become large due to the difference in cooling efficiency, as can be understood from FIG. 8, even if the pressure distribution is not so large. In the electrostatic adhering method described in this example, there are provided positive and negative electrodes on the cooling portion to which a direct current high voltage is applied to produce an electrostatic adhering force. In the electrostatic adhering method of this type, there may arise a disadvantage in that, when a substrate is treated in a plasma, the electric charge on the surface of substrate produced by irradiated ions or electrons is apt to be non-uniform, and so a current flows on the surface of substrate to damage the substrate.
Each of the conventional technologies, as described above, has the main objective of cooling a substrate efficiently. However, with an increase in integration of semiconductor devices in recent years, it is required to decrease the amount of small foreign substances, such as particles or dust and heavy metal impurities, to less than the allowable limit in the past. The same can be said for foreign substances attached on the back surface of a substrate. When the amount of foreign substances attached on the back surface of a substrate is large, there arises a disadvantage in the next process in that the foreign substances on the back surface are attached to the top surface of an adjacent substrate, or are removed first from the substrate and attached to another substrate. Therefore, decreasing the amount of foreign substances is an important problem for stabilizing the semiconductor production process or improving the yield. Attaching of foreign substances on the back surface of a substrate occurs by contacting the back surface of the substrate to another member. Therefore, a lot of foreign substances are attached to a substrate by contacting a cooling surface for the substrate.
Further, the prior art does not refer to the consideration of substrate size. Although it is mentioned that the influence upon the process is lessened by leaking cooling gas into the treating chamber, with the adhering force being as small as possible, the relation between the adhering force and the cooling gas pressure is not mentioned.
A conventional substrate holding system in a substrate etching apparatus generally employs a method in which a substrate is pressed along its periphery with hooks, as described in Japanese Patent Application Laid-Open No. 2-148837 (1990) or Japanese Patent Application Laid-Open No. 2-267271 (1990). When there is such a member contacting the surface of the substrate, problems arise in that the contact portions in the substrate are obstructed by the etching, the contacting member itself being also etched to some extent together with the substrate. As a result, the foreign substance sources, such as reaction products, are attached to the contacting member and the contacting member may be damaged, which may lead to production of foreign substances.
On the other hand, in a substrate holding method in which a substrate is held using electrostatic force (hereinafter, referred to as "electrostatic adhering"), as described in, for example, Japanese Patent Application Laid-Open No. 2-135753 (1990), a substrate is placed on an electrostatic adhering portion made of a dielectric material and a high voltage is applied to hold the substrate with an electrostatic adhering force. In this case, there is no special member to press the substrate in the periphery of the substrate. Therefore, the problem of the possibility of producing foreign substances as described in the above example is solved. However, the positional relationship between the substrate and the electrostatic adhering member is such that the substrate is placed in the uppermost position (substrate etching space side) and a step is formed in the electrostatic member such that the electrostatic adhering member comes to be placed below the substrate. When such a step exists, gas flow during etching a substrate changes abruptly at the periphery of the substrate to cause a non-uniform etching in the substrate in some cases.